Magnetic switching of charge separation lifetimes in artificial photosynthetic reaction centers

ABSTRACT

Excitation of a triad artificial photosynthetic reaction center consisting of a porphyrin (P) convalently linked to a fullerene electron acceptor (C 60 ) and a carotenoid secondary donor (C) leads to the formation of a long-lived C + -P-C 60   −  charge-separated state via photoinduced electron transfer. This reaction occurs in a frozen organic glass down to at least 8 K. At 77 K, charge recombination of C* + -P-C 60   −  occurs on the μs time scale, and yields solely the carotenoid triplet state. In the presence of a small (20 mT) static magnetic field, the lifetime of the charge-separated state is increased by 50%. This is ascribed to the effect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initially formed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four states have comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. In the presence of the field, the S and T 0  states are still rapidly interconverting, but the T +  and T −  states are isolated from the other two due to the electronic Zeeman interaction, and are not significantly populated. Under these conditions, recombination to the triplet occurs only from T 0 , and the lifetime of the charge-separated state increases. This effect can be used as the basis for a magnetically controlled optical or optoelectronic switch (e.g. AND gate).

This application is a National Stage (371) of PCT/US99/23012, filed Oct.1, 1999, which claims benefit of U.S. Provisional Application No.60/102,597, filed Oct. 1, 1998.

Financial assistance for this project was provided by U.S. Governmentthrough the National Science Foundation under Grant Number CHE-9709272;and the United States Government may own certain rights to thisinvention.

FIELD OF THE INVENTION

The present invention relates generally to the effects of magneticfields on the lifetimes of radical pairs formed generally byphotoinduced electrons transfer and the corresponding yields of theproducts of radical pair decay and more particularly to the magneticswitching of charge separation lifetimes in artificial photosyntheticreaction centers.

BACKGROUND ART

The effects of magnetic fields on the lifetimes of radical pairs formedby photoinduced electron transfer and the corresponding yields of theproducts of radical pair decay have been studied for many years.(Steiner, U. E. et al. Chem. Rev. 1989, 89, 51-147.; Steiner, U. E. etal. Photochemistry and Photophysics, Vol IV; Rabek, J. F., Ed.; CRC:Boca Raton, Fla., 1991; pp 1-130.; Hayashi, H. In Photochemistry andPhotophysics, Vol I; Rabek, J. F., Ed.; CRC: Boca Raton, Fla., 1990; pp59-136.; Grissom, C. B. Chem. Rev. 1995, 95, 3-24). These effects arisebecause the field can affect the rates of interconversion among thesinglet radical pair and the three sublevels of the triplet radicalpair. The observation of such effects requires two apparentlycontradictory conditions. In order for photoinduced electron transfer tooccur from an excited singlet state, the electronically excited electrondonor (acceptor) and the ground-state acceptor (donor) must experiencerelatively strong electronic coupling so that electron transfer cancompete kinetically with the other decay pathways available to theexcited state. However, such strong coupling generally precludes theinterconversion of the singlet and triplet radical pair states necessaryfor the development of magnetic field effects. Both conditions may besatisfied sequentially by allowing diffusional processes to bringtogether the donor and acceptor, thus promoting rapid photoinducedelectron transfer. Diffusion can then separate the radical pairs,reducing coupling and allowing singlet-triplet interconversion. Thus,magnetic field effects are generally observable in radical pair systemswherein the donors and acceptors freely diffuse in solution, or inbiradicals where the radicals are linked by flexible chains such aspolymethylene groups so that large-scale internal motions are facile.

Because of these restrictions, rigid donor-acceptor assemblies, or thosein media such as low-temperature glasses or plastics where molecularmotions are restricted, typically do not demonstrate magnetic fieldeffects on radical pairs originating from excited singlet stateprecursors. This hinders the use of such effects in the design ofmolecular-scale electronic components that must function in rigid media.The conundrum can be avoided by employing a multistep electron transferstrategy whereby the electron is moved from the primary donor to theultimate acceptor via intermediate donor-acceptor species. In this way,the electronic coupling between adjacent donor-acceptor pairs is strongenough so that each electron transfer step is rapid and can compete withother decay pathways, resulting in a high yield of the finalcharge-separated state. At the same time, the electronic couplingbetween the donor and the ultimate acceptor is small, and this can allowrapid singlet-triplet interconversions and consequently magnetic fieldeffects.

The preeminent example of this phenomenon is photosynthesis, where anumber of different magnetic field effects on reaction yields and rateshave been observed. (see: Blankenship, R. E. et al. Biochim. Biophys.Acta 1977, 461, 297-305.; Blankenship, R. E. Acc. Chem. Res. 1981, 14,163-170.; Hoff, A. J. et al. Biochim. Biophys. Acta 1977, 460, 547-554.;Hoff, A. J. Photochem. Photobiol. 1986, 43, 727-745.; Boxer, S. G. etal. J. Am. Chem. Soc. 1982, 104, 1452-1454.; Boxer, S. G. et al. J. Am.Chem. Soc. 1982, 104, 2674-2675.; van Dijk, B. et al. J. Phys. Chem. B1998, 102, 464-472). In the case of photosynthetic model systems,magnetic-field-dependent nonexponential decays of biradical states atroom temperature have been reported in diporphyrin-imide triadmolecules. (see: Werner, U. et al. J. Phys. Chem 1995, 99, 13930-13937).Small magnetic fields increased the initial rate of decay of thebiradical to the ground state by charge recombination. Magnetic fieldeffects have also been reported in dyads consisting of porphyrins linkedto viologen electron acceptors by flexible chains. (Saito, T. et al.Bull. Chem. Soc. Jpn. 1988, 61, 1925-1931.; Nakamura, H. et al. Chem.Lett. 1 987, 543-546). In these reported cases, the photoinducedelectron transfer originates from the porphyrin excited triplet state,rather than the singlet.

Recently reported (see: Liddell, P. A. et al. J. Am. Chem. Soc. 1997,119, 1400-1405.; Carbonera, D. et al. J. Am. Chem. Soc. 1998, 120,4398-4405; Gust, D. et al. In Recent Adavances in the Chemistry andPhysics of Fullerenes and Related Materials; Kadish, K. M., Rutherford,A. W., Eds.; The Electrochemical Society: Pennington, N.J., 1997; pp9-24) is the preparation and study of a carotenoid (C) porphyrin (P)fullerene (C₆₀) triad artificial photosynthetic reaction center (1),shown below, that demonstrates photoinduced electron transfer behaviorideally suited for the observation of unusual magnetic field effects.Excitation of the porphyrin moiety yields C-¹P-C₆₀, which decays byphotoinduced electron transfer to give C-P⁺-C₆₀*⁻. Secondary electrontransfer from the carotenoid to the porphyrin radical cation producesthe C*⁺-P-C₆₀*⁻ charge-separated state. This process occurs even inlow-temperature organic glasses where molecular motions and someelectron spin relaxation processes are slowed. In addition, chargerecombination yields only the carotenoid triplet state, ³C-P-C₆₀, ratherthan the molecular ground state. As discussed below, this combination ofproperties results in a lifetime for the C*⁺-P-C₆₀*⁻ charge-separatedstate on tile microsecond time scale that is increased by 50% uponapplication of a weak magnetic field.

DISCLOSURE OF INVENTION

The magnetically activated optoelectronic switch as herein described andillustrated can be used in several applications. For example, it can beconfigured as an AND logic gate, AND logic gate, or other form of logicgate. In this configuration, it is used as a molecular-scale logicelement in an optoelectronic computer processor. AND gates, either aloneor in combination, can perform all logic operations necessary forcomputer data processing. By using a gate of molecular dimensions, thesize, cost and power requirements of computer processors could bereduced, and their speed increased.

Such molecular-scale switches can also be used in optical oroptoelectronic communications applications. In this case, the lightwhich forms one of the inputs of the device can be from a fiber opticcommunication line, and the magnetic input provided by conventionalelectronic means.

A unique feature of this switch is that it is sensitive to inputs havingdifferent physical properties. One input is electromagnetic radiation inthe ultraviolet or visible wavelength range and the other input is amagnetic field. This allows information transmitted in different mediato be combined and processed.

Alternatively, the device can be configured as a computer memory elementthat can be switched between “on” and “off” binary states using acombination of light and magnetic fields. The density of data storage insuch molecular-scale memories will be much higher than in conventionalelectronic memories, and the access speed will be extremely high. Suchmemories would have to be refreshed as a function of the lifetime of thecharge-separated state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the high-energy states of C-P-C₆₀ triad 1 and theirrelevant interconversion pathways at 77 K in which the energies wereestimated from spectroscopic results at ambient temperature and fromcyclic voltammetric experiments carried out on suitable model compoundsin polar solvents at ambient temperatures. (Liddell, P. A. et al. J. Am.Chem. Soc. 1997, 119, 1400-1405). These energies have not been correctedfor any coulombic effects.

FIG. 2(a) shows the transient absorption spectrum of C*⁺-P-Q*⁻ taken3,950 ps after excitation of a ˜1×10⁻³ M solution of 1 in a2-methyltetrahydrofuran glass at 77 K with a 150-fs, 20 nJ laser pulseat 575 nm.

FIG. 2(b) shows the rise of the absorption in part (a), determined at950 nm and the smooth curve is an exponential fit to the rise data witha time constant of 770 ps.

FIG. 3 shows the decay of the transient absorption of the C*⁺-P-C₆₀*⁻charge-separated state in a 2-methyltetrahydrofuran glass at 77 K,monitored at 980 nm where the carotenoid radical cation absorbsstrongly, and the excitation was at 575 nm with a ˜5 ns laser pulse. Thelower, more rapidly decaying transient was obtained at zero magneticfield, and the upper, more slowly decaying signal at 41 mT. The smoothcurves passing through the experimental data are best fits of the datain the time window shown using three exponential components, asdescribed in the disclosure.

FIG. 4 shows the dependence of k_(avg), as defined by eq 1, on magneticfield strength at 77 K.

FIG. 5(a) shows the transient absorbance at 980 nm of a solution of 1 in2-methyltetrahydrofuran at 77 K determined at 41 mT minus thatdetermined at zero magnetic field which represents the difference of thetwo curves shown in FIG. 3.

FIG. 5(b) shows the integral of the transient absorbance at 980 nm of asolution of 1 in 2-methyltetrahydrofuran at 77 K, determined at 41 mT(upper curve) and zero field (lower curve) and represents the integralsof the two curves in FIG. 3 over time; and

FIG. 6 shows a schematic representation of the singlet biradical, thethree triplet sublevels of the biradical, and their decay andinterconversion pathways.

BEST MODE FOR CARRYING OUT THE INVENTION

Photochemistry at Ambient Temperatures. The synthesis andcharacterization of triad 1 has been previously reported. Liddell, P. A.et al. J. Am. Chem. Soc. 1997, 119, 1400-1405. The photochemistry of thetriad may be discussed with reference to FIG. 1, which shows therelevant high-energy states and decay pathways. Time-resolvedspectroscopic studies have shown that at ambient temperatures in2-methyltetrahydrofuran solution, excitation of the porphyrin moietyyields C-¹P-C₆₀, which decays in 10 ps by photoinduced electron transferto the fullerene to generate C-P*⁺-C₆₀*⁻ (step 3 in the Figure,k₃=1.0×10¹¹ s⁻¹). The fullerene excited singlet state accepts anelectron from the porphyrin to also yield C-P*⁺-C₆₀*⁻ (k₂=3.1×10¹⁰ s⁻¹).The overall quantum yield of C-P*⁺-C₆₀*⁻ by these two pathways isessentially unity. Secondary electron transfer from the carotenoid (step4) competes with charge recombination by step 7 to yield the finalC*⁺-P-C₆₀*⁻ charge-separated state. The rise time of this state is 80ps, and the overall yield is ˜0.14, as determined by the comparativemethod with the meso-tetraphenylporphyrin triplet state (φ_(T)=0.67,(ε_(T)−ε_(G))₄₄₀=6.8×10⁴ L mol⁻¹ cm⁻¹) as a standard, and using anextinction coefficient of 1.6×10⁵ L mol⁻¹ cm⁻¹ for the carotenoidradical cation. The C*⁺-P-C₆₀*⁻ state decays exponentially in 170 nsexclusively by charge recombination to produce the carotenoid tripletstate (Φ_(t)=0.13). Decay of ³C-P-C₆₀ occurs in 4.9 μs (k₁₅=2.0×10⁵s⁻¹). The application of a small (up to 41 mT) magnetic field to thesample at ambient temperatures had no effect on either the yield or thelifetime of the charge-separated state.

Photochemist at 77 K in the Absence of a Magnetic Field. Excitation of a˜1×10⁻⁴ M solution of 1 in a 2-methyltetrahydrofuran glass at 77 K witha 590 nm, 5 ns laser pulse gave rise to several transient species.(Liddell, P. A. et al. J. Am. Chem. Soc. 1997, 119, 1400-1405). Atransient absorption with a maximum at 700 nm corresponding to thefullerene triplet state was observed. The quantum yield of C-P-³C₆₀, was0.17, as determined by the comparative method, and it decayed with atime constant of 81 μs. At the excitation wavelength, ˜19% of the lightis absorbed by the fullerene moiety, and the quantum yield of triplet inmodel fullerene compounds is essentially unity. Thus, C-P-³C₆₀ ispostulated to form by normal intersystem crossing via step 10 in FIG. 1.This conclusion has been borne out by time-resolved epr experiments thatconfirm the intersystem crossing mechanism. (Carbonera, D. et al. J. Am.Chem. Soc. 1998, 120, 4398-4405).

The nanosecond time-resolved absorption experiments also reveal atransient absorption with a maximum at 965 nm corresponding to thecarotenoid radical cation of the C*⁺-P-C₆₀*⁻ charge-separated state. Thequantum yield is ˜0.10, based on total light absorbed and using theextinction coefficients reported for the tetraphenylporphyrin tripletstate and carotenoid radical cations at ambient temperatures. Transientabsorption experiments carried out under similar conditions withsubpicosecond excitation at 575 nm using the pump-probe technique gave arise time for C*⁺-P-C₆₀*⁻ of 770 ps at 77 K (FIG. 2). This rise time ispresumably the reciprocal of (k₄+k₇) at this temperature.

The decay of the carotenoid radical cation absorption of C*⁺-P-C₆₀*⁻occurs on the μs time scale, and is shown in FIG. 3. It is accompaniedby the rise of a new transient with a maximum absorbance at ˜550 nmwhose rise kinetics are identical to the decay kinetics of thecarotenoid radical cation. The new state is ascribed to ³C-P-C₆₀generated by charge recombination of the C*⁺-P-C₆₀*⁻ biradical. It isformed with a quantum yield of ˜0.07 based on ambient-temperatureextinction coefficients, which is comparable to the yield ofC*⁺-P-C₆₀*⁻, within experimental error. The decay can be fit within thenoise limits with two exponentials. The major component has a timeconstant of 10 μs, which is a typical lifetime for a carotenoid tripletstate, and the lifetime of the minor component is 83 μs. The similarityof the minor component lifetime to that for C-P-³C₆₀ suggests that itmay be due to some absorption of the fullerene triplet state at 550 nm.

The 980-nm absorption of C*⁺-P-C₆₀*⁻ at 77 K does not decay as a singleexponential, in contrast to the behavior at ambient temperatures. Inorder to compare these results with those obtained in the presence of amagnetic field, The “average” decay rate, k_(avg), for the illustratedtime window is defined by eq 1, where A, A′ and A″ are the amplitudes ofthree exponential decay components with rate constants $\begin{matrix}{k_{avg} = \frac{{Ak} + {A^{\prime}k^{\prime}} + {k^{\prime\prime}A^{\prime\prime}}}{A + A^{\prime} + A^{\prime\prime}}} & (1)\end{matrix}$

k, k′, and k″. For example, at zero field, the decay may be fitted overthe time window shown in FIG. 3 with two exponential components of 1.0μs (60% of the decay) and 2.5 μs (37%) and very slowly decayingcomponent with an amplitude of 3%. Thus, the k_(avg) value for 1 is7.5×10⁵ s⁻¹ in the absence of an applied magnetic field. Using differenttime windows yields slightly different combinations of time constantsand amplitudes.

Photochemistry at 77 K in the Presence of a Magnetic Field. Identicalexperiments at 77 K on the nanosecond time scale were carried out in thepresence of a weak (41 mT), steady-state magnetic field (B) appliedbefore excitation. The yield of C*⁺-P-C₆₀*⁻ under these conditions wasessentially identical to the yield at its B=0. The decay of thecharge-separated state, however, was significantly slower than in theabsence of a magnetic field (FIG. 3). Application of eq 1 to these datayields a k_(avg) value of 5.0×10⁵ s⁻¹ in a 41 mT field.

The magnetic field dependence of k_(avg) is shown in FIG. 4. The effecton the lifetime of the biradical saturates at a few tens of mT; thefield strength for half-saturation, B_(1/2), is 6.4 mT. The timedependence of the difference in the transient absorption of thebiradical at zero field and at saturating magnetic field effect may bequantified by subtracting the decay curve in the absence of field fromthat obtained at 41 mT. The resulting curve (FIG. 5a) shows that themaximum difference in concentration of the charge-separated states, as afunction of time, occurs 1.3 μs after the excitation pulse. The totaldifference in response of the triad to the two field strengths isconveniently illustrated and quantified by integrating the two transientabsorptions over their lifetimes (FIG. 5b).

The yield of the carotenoid triplet species, ³C-P-C₆₀, arising fromcharge recombination was determined at zero field and at saturatingfield (41 mT), and no differences outside of experimental error wereobserved. In addition, the rise kinetics of the carotenoid triplet stateat 0 mT and 41 mT were identical with the decay kinetics of thecarotenoid radical cation. Thus, the carotenoid triplet state is theonly significant product of charge recombination at all field strengthsinvestigated. The carotenoid triplet state lifetime at 77 K wasindependent of the magnetic field.

The 50% increase in the lifetime of the C*⁺-P-C₆₀*⁻ charge-separatedstate resulting from the application of a relatively weak staticmagnetic field may be explained by reference to the simple andwell-known kinetic scheme in FIG. 6, which shows the singlet biradicalstate, the three triplet biradical sublevels, and the pathways thatinterconvert and depopulate them. The rapid (770 ps) formation of theC*⁺-P-C₆₀*⁻ state at 77 K ensures that its excited state precursor isthe porphyrin first excited singlet state, rather than the triplet.Thus, C*⁺-P-C₆₀*⁻ is formed as a singlet biradical. It has beenpreviously determined by time resolved epr spectroscopy that theexchange interaction energy between the two electrons of the biradicalis very small (J=0.12 mT). In this case, the spin Hamiltonian H can bedivided into four parts, (Closs, G. L. Advan. Magn. Reson. 1974, 7,157-229). $\begin{matrix}{H = {{\beta \quad {B_{0}\left( {{g_{1}S_{1}} + {g_{2}S_{2}}} \right)}} - {J\left( {\frac{1}{2} + {2S_{1}S_{2}}} \right)} + {\sum\limits_{i}{a_{1i}S_{1}I_{i}}} + {\sum\limits_{k}{a_{2k}S_{2}I_{k}}}}} & (2)\end{matrix}$

where β is the Bohr magneton, B₀ is the applied magnetic field, g₁ andg₂ are the electronic g-factors for each radical, S₁ and S₂ are electronspin operators for the two components of the biradical, J is the scalarexchange interaction constant, I_(i) and I_(k) are nuclear spinoperators, and a_(li) and a_(2k) are the isotropic hyperfine couplingconstants of nucleus i with radical 1 and nucleus k with radical 2. Thefirst term in eq 2 accounts for the different electronic Zeemaninteractions of the two radicals with the magnetic field. The secondtern represents the scalar coupling interaction of 2J, and the last twoterms allow for the isotropic hyperfine coupling interactions ofmagnetically active nuclei (mainly protons) with the electron spins. Itis assumed that nuclei associated structurally with a given radicalcouple only with that radical; this is justified when J is very small.

The singlet biradical S (FIG. 6) can in principle decay by chargerecombination (k₀, which is identical to k₈ in FIG. 1), or interconvertwith the T₀ (k₀), or T₊ and T− (k_(b)) states. The three tripletsublevels can interconvert via k_(c), and each can undergo chargerecombination to yield the carotenoid triplet state with a rate constantk_(T).

At zero applied field, the first term in eq. 2 vanishes, and theenergies of all four states in FIG. 6 are nearly degenerate when theexchange interaction energy is very small, as in 1. It is postulatedthat as a result, interconversion among all four states via hyperfinecoupling interaction (HFI)-induced intersystem crossing is rapid on thetime scale of charge recombination. The resulting equilibrium leads to apopulation that is 25% singlet biradical and 75% triplet. Theexperimental results show that this population decays only within thetriplet manifold (k_(S)<<k_(T)), yielding ³C-P-C₆₀ as the sole product.Under these conditions, and assuming all k_(T) values to be equal, thedecay of the charge-separated state is given by $\begin{matrix}{\frac{\lbrack{BR}\rbrack}{t} = {\frac{- {3\lbrack{BR}\rbrack}}{4}k_{T}}} & (3)\end{matrix}$

where [BR] is the total concentration of the biradical. Thecharge-separated state will decay by step 9 in FIG. 1 with an observedfirst order rate constant k₉=3k_(T)/4.

At saturating applied magnetic field strengths, the Zeeman interactionof the spins with the external magnetic field (first term in eq 2)removes the near-degeneracy of the triplet sublevels, increasing theenergy of T₊ and decreasing that of T− by an equal amount. Under theseconditions, the mixing between T₀ and S is much stronger than the mixingamong any other electronic states. At 77 K, it is postulated that on thetime scale of biradical recombination, the interconversion of the S andT₀ levels is still rapid. However, interconversion between these statesand T₊ and T− by any relaxation mechanism is now much slower than chargerecombination, and the T₊ and T− states are in effect isolated from theother two. Under these conditions, C*⁺-P-C₆₀*⁻, which is born as asinglet biradical, rapidly equilibrates with T₀, but T₊ and T− are notsignificantly populated. When the field is present, the decay of thebiradical is given by $\begin{matrix}{\frac{\lbrack{BR}\rbrack}{t} = {\frac{- \lbrack{BR}\rbrack}{2}k_{T}}} & (4)\end{matrix}$

and the first order decay constant k₉ is k_(T)/2. Thus, the observed k₉at zero field will equal 1.5 times k₉ at a saturating field strength.This result corresponds to what is observed experimentally, as k_(avg)at zero field is indeed 1.5 times that at a saturating field.

It was noted that the decays at 77 K are not single exponentials. Thisis ascribed, for the most part, to the likelihood that in the frozenmedium, a variety of molecular microenvironments and a variety ofslightly different molecular conformations exist, and that these haveslightly different recombination rates. Thus, although the data may befit, within experimental error, by two exponentials and a small baselineoffset, the true kinetic description is likely better described as adistribution of decays. In this connection, it is interesting to notethat both k and k′ in eq 1 are larger by a factor of ˜1.5 in the absenceof a magnetic field. Thus, the molecules in the great majority of thepostulated microenvironments respond in the same way to the presence ofthe field.

It is worthwhile to briefly examine a few of the postulates underlyingthe above explanation. As determined by epr spectroscopy, the coupling Jbetween the electron spins in C*⁺-P-C₆₀*⁻ is +0.12 mT. This means thatthe T₀ state lies 2.7×10⁻⁸ eV below the singlet state. The zerofieldsplitting between the triplet states must be of this general magnitudeas well. At 77 K, thermal energies are ˜6.6×10⁻³ eV. Thus, theassumption that when in equilibrium at zero field, the four energylevels are essentially equally populated is justified.

It is generally found that the dominant mechanism for singlet-tripletinterconversion at zero and low fields is HFI-induced intersystemcrossing. (Steiner, U. E.; Ulrich, T. Chem. Rev. 1989, 89, 51-147.;Steiner, U. E. et al. Photochemistry and Photophysics, Vol IV; Rabek, J.F., Ed.; CRC: Boca Raton, Fla., 1991; pp 1-130.; Hayashi, H.Photochemistry and Photophysics, Vol I; Rabek, J. F., Ed.; CRC: BocaRaton, Fla., 1990; pp 59-136.; Grissom, C. B. Chem. Rev. 1995, 95,3-24). At zero field, the four spin levels are essentially isoenergetic,and HFI-induced intersystem crossing rapidly equilibrates all fourstates. As the magnetic field strength increases and the Zeemaninteraction splits the energies of T₊ and T⁻ away from those of T₀ and S(FIG. 6), HFI-induced intersystem crossing to T₊ and T⁻ becomes slower.Finally these two states are no longer populated, and the magnetic fieldeffect saturates. Weller and coworkers have shown (Weller, A. et al.Chem. Phys. Lett. 1193, 96, 24-27) that for a series of organic radicalpairs, the magnetic field value at half saturation due to HFI, B_(1/2)is given by: $\begin{matrix}{B_{\frac{1}{2}} = {2\frac{B_{1}^{2} + B_{2}^{2}}{B_{1} + B_{2}}}} & (5)\end{matrix}$

where B₁ and B₂ refer to the energies of the hyperfine interactionsbetween the nuclear spins and unpaired electron spins on each radical.These may be determined from the values of the isotropic hyperfinecoupling constants a_(ik). For the current example, where tile dominanthyperfine interactions are with protons (I=½), the value of B_(i) isgiven by: $\begin{matrix}{\left. {B_{i}\left( {\sum\limits_{k}{{I\left( {I + 1} \right)}a_{ik}^{2}}} \right)} \right)^{\frac{1}{2}} = \left( {\sum\limits_{k}\frac{3a_{ik}^{2}}{4}} \right)^{\frac{1}{2}}} & (6)\end{matrix}$

In the case of triad 1, the fullerene moiety is expected to contributenegligibly because there are few, if any, hyperfine couplinginteractions with protons. This is consistent with the narrow epr linewidth of this radical (˜0.1 mT). (Zoleo, A. 1995. Dissertation,Universita di Padova). The large number of hyperfine couplings in thecarotenoid radical cation, which lead to a broad epr signal, areexpected to dominate B_(1/2). The proton hyperfine coupling constantsfor β-carotene have been determined. (Piekara-Sady, L. et al. Chem.Phys. Lett. 1991, 186, 143-148). Using these values to estimate thosefor the carotenoid of 1, then B_(car) is 3.4 mT, and eq 5 yields aB_(1/2) value of 6.8 mT, which is in good agreement with theexperimental value of 6.4 mT (FIG. 4). This result is consistent withthe assignment of HFI-induced intersystem crossing as the majorintersystem crossing mechanism.

In the presence of magnetic fields, the spin rephasing (Δg) mechanismcan also interconvert the S₀ and T₀ states. This mechanism relies on thedifference in precession frequencies of the electron spins in thebiradical (Δg) that arises from differences in chemical environments(first term in eq 2). The time for interconversion of the S₀ and T₀states by this mechanism (Δω) is given (Grissom, C. B. Chem. Rev. 1995,95, 3-24) by:

Δω=ΔgβB/  (7)

The g values for the carotenoid radical cation and fullerene radicalanion are 2.0027 and 1.998, respectively. (Zoleo, A. 1995. Dissertation,Universita di Padova.; Grant, J. L. et al. J. Am. Chem. Soc. 1988, 110,2151-2157.; Hasharoni, K. et al. J. Am. Chem. Soc. 1990, 112,6477-6481). At the field where the magnetic field effects in 1 saturate,˜0.020 mT, 1/Δω equals 120 ns. Thus the Δg mechanism could have someeffect on S₀−T₀ interconversion at the higher magnetic field strengths,although the HFI mechanism dominates overall, as discussed above.

Spin-orbit coupling can also in principle lead to interconversion amongspin states in biradicals. Spin-orbit coupling is most important forheavy atoms, and interconversion by spin-orbit coupling is expected tobe too slow to be significant in triad 1 at 77 K. The fact that amagnetic field effect is observed at all, and that it saturates at lowmagnetic fields shows that rapid interconversion of all spin states byspin-orbit coupling or any other mechanism is not, in fact, occurringunder these conditions.

At 298 K, no effect of small magnetic fields on the lifetime ofC*⁺-P-C₆₀*⁻ is observed in organic solvents. Thus, under theseconditions, rapid equilibration among the various spin sublevels must beoccurring even in the presence of the field. It is possible thatproperties of the fullerene radical anion contribute to the highequilibration rate.

The treatment above assumes that all singlet-triplet interconversionoccurs in the C*⁺-P-C₆₀*⁻ biradical. Given the 770 ps rise time of thisstate at 77 K, it is conceivable that some interconversion could alsohappen in the precursor C*⁺-P-C₆₀*⁻, although the radicals arepresumably much more strongly coupled in this state. If this occurs, ithas no significant consequences. The yield of C*⁺-P-C₆₀*⁻ is essentiallythe same at 77 K and ambient temperature, where the rate of formation ofC*⁺-P-C₆₀*⁻ is much too fast to permit intersystem crossing in theprecursor biradical. The yield of C*⁺-P-C₆₀*⁻ at 77 K is alsoindependent of magnetic field strength. This suggests thatsinglet-triplet interconversion in C*⁺-P-C₆₀*⁻ is not affecting thecompetition between steps 4 and 7 in FIG. 1.

Triad 1 provides a unique opportunity to observe a straightforwardinfluence of weak magnetic fields on radical pair recombination, and theexpected 50% increase in the lifetime of the charge-separated state.There are several reasons for this situation. In the first place, 1generates the charge-separated state via a multistep electron transferstrategy that produces a long-lived charge-separated state in highyield, but with only a small exchange interaction between the radicals.This weak coupling permits singlet-triplet interconversion by hyperfineinteractions of the electron spins with nuclear magnetic moments.Presumably, these are mainly the hydrogen atoms on the carotenoid chain.Secondly, charge separation occurs in a rigid glass at 77K (electrontransfer occurs down to at least 8 K in 1), and this establishes theconditions for rapid equilibration of the S and T₀ states with nopopulation of T₊ and T⁻ in the presence of the magnetic field. Finally,charge recombination yields exclusively the carotenoid triplet state. Ifrecombination occurred only to the singlet ground state, the lifetime ofthe charge-separated state would be reduced by the magnetic field.

As mentioned above, there are numerous examples of magnetic fieldeffects on reaction yields in mobile radical ion pairs (MARY effect).(see: Steiner, U. E. et al. Chem. Rev. 1989, 89, 51-147.; Steiner, U. E.et al. Photochemistry and Photophysics, Vol IV; Rabek, J. F., Ed.; CRC:Boca Raton, Fla., 1991; pp 1-130.; Hayashi, H. Photochemistry andPhotophysics, Vol I; Rabek, J. F., Ed.; CRC: Boca Raton, Fla., 1990; pp59-136.; Grissom, C. B. Chem. Rev. 1995, 95, 3-24). Examples with rigidsystems are less common. Effects related to those reported here havebeen observed in photosynthetic reaction center preparations, wherelarge-scale motions within the protein matrix are restricted. Inreaction centers of Rhodobacter sphaeroides excitation of the specialpair of bacteriochlorophyll molecules is followed by photoinducedelectron transfer via a bacteriochlorophyll monomer to abacteriopheophytin. An electron then migrates to a quinone acceptorQ_(A), and finally to a second quinone Q_(B). If Q_(A) is pre-reduced sothat it cannot act as an acceptor, a long-lived radical pair consistingof the special pair radical cation and bacteriopheophytin radical anionis formed. The singlet radical pair can recombine to yield the groundstate, or undergo intersystem crossing to the triplet radical pair,which decays to yield the triplet state of the special pair. In thepresence of a magnetic field of 50 mT, the yield of triplet chromophoredecreases by ˜50%. (Blankenship, R. E. et al. Biochim. Biophys. Acta1977, 461, 297-305.; Hoff, A. J. et al. Biochim. Biophys. Acta 1977,460, 547-554). The genesis of this effect is that at zero field, thesinglet radical pair can interconvert rapidly with the three tripletsublevels by hyperfine interactions. In the presence of the field,interconversion only occurs between the singlet and T₀ levels, asdiscussed above, and therefore charge recombination to the ground stateis enhanced. Other effects are observed at much higher field strengths.

A magnetic field effect on quantum yields such as that found in reactioncenters can only be observed if radical pair recombination can occur bytwo competing mechanisms. In triad 1, this is precluded because chargerecombination yields only the carotenoid triplet state, but lifetimeeffects are observed. A somewhat related situation has been observed inmodified bacterial reaction centers at temperatures below ˜70 K. Underthese conditions, electron transfer proceeds to yield a radical pairconsisting of the special pair radical cation and the Q_(A) radicalanion, but further electron transfer is blocked. Charge recombinationoccurs only from the singlet radical pair to yield the ground state, asthe special pair triplet state is energetically inaccessible. At fieldsof 50 mT, the initial part of the nonexponential decay of the radicalpair to the ground state shows an increase in decay rate, attributableto decreased interconversion of the S and T₀ states with the T₊ and T⁻states.

Magnetic field effects at ambient temperatures have been reported intriad photosynthetic model systems consisting of two covalently linkedporphyrins, one of which bears an aromatic imide electron acceptor.(see: Werner, U. et al. J. Phys. Chem 1995, 99, 13930-13937). The decayto the ground state of the biradical resulting from photoinducedelectron transfer is biphasic, and the rate of the initial part of thedecay increases in the presence of a very small (5 mT) magnetic field.This effect, which is quite different from those observed for 1, isascribed to an initial decay of the singlet biradical to both the groundstate and the triplet manifold, with comparable rate constants.Eventually a dynamic equilibrium is established, giving rise to a slowercomponent. The magnetic field decreases the rate of intersystemcrossing, thus affecting the rate constants and enhancing singlet decayto the ground state at early times. In these systems, magnetic fieldeffects are not observed when the charge separation lifetime is longerthan a few hundred ns.

The magnetic field effects observed for 1 are potentially useful in twoways. In the first place, the enhanced lifetime of a charge-separatedstate induced by the magnetic field could in principle be of value insolar energy conversion applications of artificial reaction centers. Alonger lifetime could increase the quantum yield of useful energyconversion reactions by slowing down charge recombination. Of course,this is only a 50% effect, and is observable only at low temperatures in1.

The ability to significantly slow charge recombination by application ofa small field could be the basis of a magnetically controlled optical oroptoelectronic switch or logic gate. The change in absorption of thecarotenoid radical cation at 980 nm, monitored either at the time ofmaximum difference in the curves with the field on and the field off(FIG. 5a), or as the integral of the absorptions over their lifetimes(FIG. 5 b), is much larger than the noise level in these experiments.Appropriate monitoring of this transient absorption with a probe lightbeam and a detector with a suitable threshold would allow implementationof an AND gate-like function. In its unperturbed resting state, acollection of triads would have no detectable absorption at 980 nm, asno light is absorbed by the ground state in this spectral region.Switching on the magnetic field would not generate an absorption.Excitation in the absence of a magnetic field would generate a signalbelow the detection threshold. However, excitation in the presence ofthe field would produce a measurable signal, and therefore a change inthe state of the device. Thus, an output signal would be produced onlyin the simultaneous presence of two inputs, the magnetic field and theexcitation pulse, as required for an AND gate. A similar device with anelectrical readout could be devised if electron transfer from thecharge-separated state of the triad to an external circuit competed withdecay to the carotenoid triplet state. The electrical signal would belarger in the presence of the magnetic field, when recombination to thetriplet was slowed.

Experimental Section

Transient absorption measurements on the picosecond time scale were madeusing the pump-probe technique. The sample was dissolved in purified2-methyltetrahydrofuran and the resulting ˜0.001 M solution was placedin a cuvette having a 2-mm path length in the beam area and stirred. Forlow-temperature measurements, a closed-circuit helium refrigerator fromAPD Cryogenics Inc. equipped with a Model 330 temperature controllerfrom Lake Shore Cryogenics was used. Excitation was at 590 nm with150-200 fs, 30 μJ pulses at a repetition rate of 540 Hz or with 100-150fs, 5 μJ pulses at a repetition rate of 1000 Hz. The signals from thecontinuum-generated white-light probe beam were collected by an opticalspectrometric multichannel analyzer with a dual diode array detectorhead. (Lin, S. et al. Biophysical Journal 1994, 66, 437-445).

For nanosecond transient absorption measurements, similar solutions wereplaced in a cuvette with a 1-cm path length. For measurements at ambienttemperatures, the sample was deoxygenated by purging with argon. Thetransient absorption measurements were made with excitation from anOpotek optical parametric oscillator pumped by the third harmonic of aContinuum Surelight Nd:YAG laser. The pulse width was ˜5 ns, and therepetition rate was 10 Hz. The detection portion of the spectrometer hasbeen described elsewhere. (Davis, F. S. et al. Rev. Sci. Instrum. 1987,58, 1629-1631).

Magnetic fields were generated using Helmholtz coils. The sample, sealedin a cuvette, was immersed in liquid nitrogen in a clear glass dewarflask. The magnetic field was measured at the sample using a BellGaussmeter (calibrated to ±3%).

From the foregoing it is readily apparent that a new and usefulembodiment of the present invention has been herein described andillustrated which fulfills all of the aforestated objects in aremarkably unexpected fashion. It is, of course, understood that suchmodification, alterations and adaptations as may readily occur to theartisan confronted with this disclosure are intended within the spiritof this invention which is limited only by the scope of the claimsappended hereto.

What is claimed is:
 1. A magnetically activated optoelectronic logicgate comprising: a. means for receiving and storing ultraviolet orvisible wavelength and magnetic field signals wherein the means furthercomprises a photoreactive molecule capable of forming transient specieswhen activated by said electromagnetic radiation signal, the lifetime ofsaid transient species being modified in the presence of a magneticfield; and b. means for selectively accessing said stored signals todeliver said selected signals for signal processing.
 2. Theoptoelectronic logic gate of claim 1 wherein said photoreactive moleculecomprises an electron donor, an electron acceptor and a chromophore. 3.The optoelectronic logic gate of claim 1 wherein said electron donor isa carotene.
 4. The optoelectronic logic gate of claim 1 wherein saidelectron acceptor is a fullerene.
 5. The optoelectronic logic gate ofclaim 1 wherein said chromophore is a porphyrin.
 6. The optoelectroniclogic gate of claim 1 wherein said photoreactive molecule comprises acarotene, a fullerene and a porphyrin.
 7. The optoelectronic logic gateof claim 1 wherein said transient species is a long-livedcharge-separated molecule capable of decaying by radical pairrecombination to yield the triplet state.
 8. The optoelectronic logicgate of claim 3 wherein the lifetime of said transient species isextended by application of a magnetic field.
 9. A means for generatingmagnetic field signals to the transient species of claim 1 comprises aHelmholtz coil in magnetic contact with said transient species.
 10. Theoptoelectronic logic gate of claim 1 comprising in addition a means forselectively controlling the time period during which said magneticsignals are generated.
 11. The optoelectronic logic gate of claim 1wherein said means for selectively accessing and delivering said storedsignals for signal processing comprises: a. means for activating saidtransient species; b. means for applying a magnetic field to saidtransient species for a selected period of time; c. means fortransmitting an optoelectronic radiation signal through said transientspecies; d. means for receiving the transmitted optoelectronic radiationsignal in the presence of the magnetic field, wherein processing of saidtransmitted electromagnetic radiation signal comprises comparison ofsaid received signal to a threshold value to provide a Boolean yes/nosignal.
 12. The optoelectronic logic gate of claim 7 wherein saidelectromagnetic radiation signal is light of a known wavelength and thedifference between incident and transmitted signal is the absorbance orper cent transmission of said light.
 13. The optoelectronic logic gateof claim 7 wherein said electromagnetic radiation signal is electronicand said difference between said incident and transmitted signal is theconductance or capacitance of said electronic signal.
 14. A digitaldevice comprising the magnetically controlled logic gate of claim 1wherein processing of said transmitted electromagnetic radiation signalcomprises comparison of said signal to a threshold value to provide aBoolean yes/no signal.
 15. A magnetically controlled optoelectroniclogic gate of claim 1 comprising: a. a photoreactive molecule capable offorming transient species when activated by an electromagnetic radiationsignal, the lifetime of said transient species being altered in thepresence of a magnetic field; b. means for activating said photoreactivemolecule to form said transient species; c. means for delivering amagnetic field signal to said transient species for a selected period oftime; d. generator means for transmitting an optoelectronic signalthrough said transient species; e. monitor means for detecting thetransmitted optoelectronic signal in the presence and absence ofmagnetic field signal; and f. output means for delivering signals fromsaid monitor means to a signal processor.
 16. The logic gate of claim 11in a computer processor.